Obesity, a chronic disorder affecting one third of the U.S. adult population, is the second leading cause of preventable death and a major contributor to increased health care costs. Even a modest weight loss can ameliorate complications and improve longevity. Although behavioral weight loss results in good short-term success, it is followed by an extremely high rate of recidivism. The primary aim of the present study is to promote long-term weight loss by focusing on the use of a lacto-ovo-vegetarian (LOV) eating plan as a dietary strategy, and also on the role of treatment preference. Clinical studies show that vegetarian eating plans can be followed for sustained periods and can produce significant and sustained weight loss as well as numerous health benefits. Pilot data showed that a vegetarian plan leads to acceptable weight loss. There is also evidence to suggest that use of treatment preference leads to improved adherence and reduced attrition. The proposed study will test if giving participants their preference leads to improved adherence, reduced attrition, improved long-term weight loss. Preference will be crossed with 2 treatment options: standard behavioral treatment and standard behavioral treatment + lacto-ovo-vegetarian eating plan (SBT + LOV). Participants (n=168) will be randomized to 1 of 2 conditions: Treatment Preference - Yes or No. Individuals within Preference-Yes will be assigned to their treatment choice (SBT or SBT + LOV), those in Preference-No will be randomized to 1 or the 2 treatment conditions : SBT or SBT + LOV. All subjects will receive treatment for 12 months and will complete assessments at baseline, 6, 12, and 18 months. The primary aim will test the interaction between treatment assignment and treatment preference and its effect on the primary outcome, weight change from baseline to 18 months. Secondary outcomes include adherence and serum lipids. This study represents an innovative approach to a serious issue, the failure to maintain long-term weight loss. It will provide important information about the efficacy of a vegetarian eating plan in the treatment of obesity, and will address the criticism of vegetarian studies that suggest that patient self-selection is a critical factor in their reported successful outcomes. Additionally, the study will provide information on the role of treatment preference, its effect on behavioral and clinical outcomes, and participants' characteristics by treatment preference.